For the recording method by means of an inkjet printer which is one of the typical methods among various color recording methods, various methods for discharging ink have been developed, and in any of the methods, ink droplets are generated and adhered onto various record-receiving materials (such as paper, film and cloth) to perform recording. This method has been rapidly prevailing lately and is expected to continue growing remarkably in the future because of features such as quietness without noise generation due to no direct contact of the recording head with a record-receiving material and as easiness in downsizing, speedup and colorization. Conventionally, as an ink for fountain pens, felt-tip pens or the like and as an ink for inkjet recording, water-based inks where a water-soluble dye is dissolved in an aqueous medium have been used, and in these water-based inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an inkjet nozzle. These inks are required to provide recorded images with a sufficient density, not to clog at a pen tip or a nozzle, to dry quickly on a record-receiving material, to bleed less, to have an excellent stability in storage, and so on. In addition, recorded images formed are required to have fastnesses such as water fastness, moisture fastness, light fastness and gas fastness.
In an inkjet printer, nozzle clogging often occurs because a coloring matter is precipitated as crystals when evaporation of the water in the ink near the nozzle leads to the state where the water content is smaller and the solvent and additive content is larger. Therefore, it is one of the very important required performances that crystal precipitation is hardly occurred even in such a condition. For this reason, a coloring matter having a higher water-solubility is required which is free from crystal precipitation even in the above condition.
In addition, in order to record images or character information on a color display of a computer in color by an inkjet printer, subtractive color mixing of 4 color inks of yellow (Y), magenta (M), cyan (C) and black (K) is generally used. By this method, images are recorded in color. In order to reproduce, as faithfully as possible, images expressed by additive color mixing of red (R), green (G) and blue (B) on CRT (cathode ray tube) displays and the like through images by subtractive color mixing, it is desired that coloring matters used in ink, particularly each of Y, M and C has a hue as close to each standard as possible and also vividness. In addition, it is required that the ink to be used for them are stable in storage for a long period of time, and that images printed in the above manner have a high concentration and are also excellent in fastnesses such as water fastness, moisture fastness, light fastness, and gas fastness. The gas fastness herein means durability against the phenomenon where oxidizing gases having oxidizing effect, such as nitrogen oxide gas, ozone gas, and the like, which exist in the air are reacted with a coloring matter (dye) of a recorded image on or in a recorded paper, resulting in discoloration or fading of printed images. In particular, the ozone gas is regarded as a main causative matter promoting the color fading phenomenon of inkjet recorded images. This discoloration or fading phenomenon is a characteristic of inkjet images and therefore improvement of ozone gas fastness is an important technical challenge in this field.
In order to obtain photo image quality, some inkjet special papers as one of recording papers are provided with an ink receiving layer on the surface thereof. For this ink receiving layer, porous white inorganic substance is often used so as to dry the ink quickly and reduce ink blurring in high image quality. Discoloration or fading by ozone gas prominently appears particularly on such a recording paper. As is described in Non-Patent Literature 1, NOx gas and SOx gas as an oxidizing gas other than ozone gas are also regarded to have a large effect on discoloration phenomenon of printed matters. In particular, NO2 gas exists in the atmosphere in a relatively large amount and therefore the effect thereof cannot be ignored. With recent spread of digital cameras and color printers, there are also more opportunities at home to print images obtained using a digital camera and the like into photo image quality, and image discoloration by these oxidizing gases in the air during storage of print matters obtained is often regarded as a problem together with ozone gas fastness.
C.I. (color index) Direct Yellow 132 is cited as a yellow coloring matter for an inkjet which is excellent in water-solubility and vividness. Use examples thereof are disclosed in Patent Literatures 1 to 3.
In addition, an azo-based yellow coloring matter having high fastnesses for inkjet recording is disclosed in Patent Literature 4.
Non-Patent Literature 1: Japan Hardcopy, 2004, Article Collection, pp. 73 to 80
Patent Literature 1: JP H 11-70729
Patent Literature 2: JP 2000-154344, Examples A1 to 5
Patent Literature 3: JP 2003-34763, Table 1-1 (Example 4)
Patent Literature 4: JP 2006-152244